Not applicable.
This invention relates to systems using vertical cavity, surface emitting lasers (VCSELs) having integrated MEMS (micro-electromechanical) wavelength tuners to interrogate optical sensors such as fiber and planar Bragg gratings, etalons, characteristic absorption or reflection sensors such as bandgap semiconductors and surface plasmon resonance sensors sensitive to physical, chemical and biological stimuli and, more particularly, to specific system configurations for use with such Bragg grating, etalon, absorption/reflection and surface plasmon resonance sensing devices.
Fiber optic sensors employing measurements of the shift of wavelength position of a sensor spectral peculiarity (e.g., maximum, minimum, slope or some other function) under the influence of a physical stimulus are well known to those skilled in the art. The examples of such sensors include Bragg grating-based strain, pressure, temperature and current (via the associated magnetic fields) sensors, surface plasmon resonance (SPR) biological and chemical sensors, semiconductor absorption band-edge based fiber-optic sensors and Fabry-Perot (FP) etalon pressure, temperature and sensors. To date, the utilization of such sensors has been retarded in the marketplace because of many well known problems, including e.g. the susceptibility of simple, inexpensive sensing systems to optical noise and the great expense of most of the solutions found to overcome said susceptibility. It will be revealed that combining a new type of laserxe2x80x94a vertical cavity, surface emitting laser (VCSEL) with an integrated microelectromechanical (MEMS) tuning mechanismxe2x80x94as an interrogating instrument with sensors of many different types will enable new, less expensive and more reliable classes of optical sensor systems.
Generally, a Bragg grating is a series of optical elements that create a periodic pattern of differing indices of refraction in the direction of propagation of a light beam. A Bragg grating can be formed in an optical fiber by exposing ultraviolet sensitive glass (usually germanium doped fiber) with an ultraviolet (UV) beam that varies periodically in intensity. This is usually accomplished by means of an interference pattern created by a phase mask or split beam, such as with a Lloyd""s mirror apparatus. Planar Bragg gratings are created by exposing a xe2x80x9cphotoresistxe2x80x9d of any of a number of types through a phase shift or other type of mask, or they can be written directly with an electron beam. Light reflections caused by the periodic index of refraction pattern in the resulting grating interfere constructively and destructively. Since the refractive index contrast between UV-exposed and unexposed sections of fiber is small but the number of sections is very large, the reflected beam narrows its spectrum to a very sharp peakxe2x80x94as narrow as a fraction of a nanometer in spectral width. It can also be arranged by means of a phase shift design that the reflected peak can contain within it an even narrower xe2x80x9cvalleyxe2x80x9d of absorption, e.g. as narrow as a few picometers in spectral width. Conversely, the transmitted portion of the light beam exhibits complimentary spectral power characteristics, i.e., a broader valley with a narrower peak within it.
It is known that Bragg gratings patterned into optical fibers or other waveguides may be used to detect physical stimuli caused by various physical parameters, such as, for example, strain, pressure, temperature, and current (via the associated magnetic fields) at the location of the gratings. See e.g. U.S. Pat. Nos. 4,806,012 and 4,761,073 both to Meltz, et al; U.S. Pat. No. 5,380,995 issued to E.
Udd; U.S. Pat. No. 6,024,488 issued to J. Wu; and the publication authored by Kersey, A. D., et.al. [10th Optical Fiber Sensors Conference, Glasgow, October 1994, pp.53-56].
Generally, in one exemplary such sensor, the core and/or cladding of the optical fiber (or planar waveguide) is written with periodic grating patterns effective for selectively reflecting a narrow wavelength band of light from a broader wavelength band launched into the core (waveguide layer in the waveguide). The spectral positions of sharp maxima or minima in the transmitted and reflected light intensity spectra indicate the intensity of strain, temperature, pressure, electrical current, or magnetic field variations at the location of the grating. The spectral positions change based on the grating period or the indices of refraction, or both. These can be affected by various environmental physical stimuli such as temperature and pressure.
Frequently, more than one stimulus or physical parameter affects the sensors at the same time. Compensation is often designed into the sensor or the measurement technique for all the variables but one. This can be accomplished by many physical, optical and electronic techniques known in the art. The typical sensitivity limits of fiber grating sensors in the current art are about 0.1xc2x0 C. and/or 1 microstrain, respectively.
Exemplary advantages of a spectral shift method of sensor interrogations include the high accuracy of wavelength determination (akin to the advantages of measuring electrical frequency instead of magnitude) and immunity to xe2x80x9coptical noisexe2x80x9ddue to fluctuations in fiber transmission amplitude (microbending losses, etc.). The spectral shift method also allows the multiplexing of many sensors on the same fiber via wavelength dependent multiplexing techniques (WDM), e.g., dividing the total wavelength band into sections dedicated to individual sensors.
The precision, dynamic range and multiplexing capabilities of the optical sensor interrogation techniques can be defined in part by the spectral power of the light source, especially in cases in which a broadband source is used. The LEDs, SLDs (superluminescent diodes) and various lamps usually used provide spectral power that can be too small when divided into nanometer-sized segments. This limits critical parameters such as the magnitude of the reflected peak available to the optical sensor, causing lower than desirable signal to noise ratios. Another technique, the use of a conventional laser diode tuned with a motorized external cavity, electrical current or temperature mechanisms is more effective because all the power of the laser is contained in a narrow beam as it is tuned across the spectrum. Several techniques have been proposed: see for example Froggatt, (U.S. Pat. No. 5,798,521). The use of a conventional laser diode tuned with electrical current has been proposed by Dunphy et al. (U.S. Pat. No. 5,401,956); and the use of a tunable fiber laser has been proposed by G. A. Ball et al. [J. of Lightwave Technology, vol. 12, no. 4, April 1994 p. 700].
When using a scanning laser technique, an inexpensive detector and electronics system can simply determine the wavelength at the peak (or null) of the reflected (or transmitted) light intensity against a known wavelength reference. However, past approaches are generally too expensive, too slow, too unstable or too inaccurate to have a wide range of practical applications.
Laser diodes tuned with current, while inexpensive and faster than thermal methods, generally can suffer from narrow tuning wavelength spansxe2x80x94which limits practical applications to only time division-multiplexed (TDM) Bragg sensors. Such lasers are completely unusable in surface plasmon or semiconductor absorption edge shift sensors. Broadband light source methods utilize inexpensive light sources, but use a spectrometer to read the signals (an optical spectrum analyzer may cost as much as $35,000). This technique is generally most practical when many sensors are multiplexed on the same fiber. Still, spectrometers are temperamental and not well suited to field use. The lasers tuned with external cavities that are now in use, on the other hand, typically are more expensive than spectrometers, but have the advantage of using an inexpensive detector. In addition, such lasers are typically slow to tune (such as 100 nm/sec) and may be even more delicate than spectrometers. Scanning (or tuning) speed is especially important in applications in which absorption and polarization related noise are significant because of negative effects on the signal-to-noise ratio (SNR).
In contrast, mass produced MEMS-tunable VCSELs, configured as sensing instruments, are expected to cost at least an order less than prior art lasers and be at least two orders of magnitude faster than prior art lasers.
Surface plasmon resonance-based sensors for biological and/or chemical monitoring are well known to those skilled in the art. Generally, surface plasmon waves are electromagnetic waves that may exist at the boundary between a metal and a dielectric (hereinafter referred to as the xe2x80x9csamplexe2x80x9d), Such waves can be excited by light that has its electric field polarized parallel to the incident plane (i.e., transverse magnetic (TM) polarized). When the parallel component of the propagation constant of the incident light equals the real part of the surface plasmon wave propagation constant, the incident light resonantly excites the surface plasmon waves, and a fraction of the incident light energy is transferred or dispersed to surface plasmon resonance (xe2x80x9cSPRxe2x80x9d). This dispersion of energy depends on both the dielectric constant of the metal and that of the sample in contact with the metal. By monitoring the resonance wavevector of the metal/sample interface, the dielectric constant of the sample (gas or solution) may be obtained. Alternatively, if the sample is contaminated by a chemical species, dielectric constant measurements may provide the concentration of the chemical species in the sample. The typical SPR spectral minimum is at least two orders of magnitude wider than the typical Bragg grating minimum or maximum.
Traditionally, SPR has been measured using the Kretschmann configuration (Kretschmann and Raether, Z. Naturforsch. Teil A 23:2135-2136, 1968). In this configuration, a thin layer of highly reflective metal (such as gold or silver) is deposited on the base of a prism. The metal surface is then contacted with the sample, and the SPR reflection spectra of the sample is measured by coupling TM polarized, monochromatic light into the prism and measuring the reflected light intensity as a function of the angle of incidence. The angle of minimum reflective intensity is the resonance angle at which maximum coupling occurs between the incident light and the surface plasmon waves. This angle, as well as the half-width of the resonance spectrum and the intensity at the angle of minimum reflective intensity, may be used to characterize or sense the sample that is in contact with the metal surface (e.g. Fontana et al., Applied Optics 27:3334-3339, 1988).
Optical sensing systems have been constructed based on the Kretschmann configuration described above. Such systems utilize the sensitivity of SPR to changes in the refractive indices of both bulk and thin film samples, as well as to changes in the thickness of thin films. These systems, in conjunction with appropriate chemical sensing layers, have led to the development of a variety of SPR-based chemical sensors, including immunoassay sensors (e.g., Liedberg et al., Sensors and Actuators 4:299-304, 1983; Daniels et al. [Sensors and Actuators 15:11-17, 1988]; Jorgenson et al., [IEEE/Engineering Medicine and Biology Society. Proceedings 12:440-442, 1990]), gas sensors (e.g., Liedberg et al., ibid, Gent et al., [Applied Optics 29:2843-2849, 1990]), and liquid sensors (e.g., Matsubaru et al., [Applied Optics 27:1160-1163, 1988]). An SPR sensor usually utilizes the wavelength of minimum amplitude as a function or angle of reflection. However, the shape of the minima can be modified if an additional polarizer and phase plate (or retarder) are introduced between the sensor and detector at some predetermined angle with respect to the polarization of light illuminating the SPR sensor (Homola J., et al., Sensors and Actuators B, B51 (1-3), August 1998, p. 331, Kabashin A. V. et al., Sensors and Actuators B, B54 (1-2), January 1999, p.51). This modification is due to phase and polarization peculiarities near the surface plasmon resonance excitation conditions. Moreover, minima can be transformed into maxima (Homola, ibid), which has the potential for increasing the resolution of SPR sensors
While the Kretschmann configuration for SPR-based chemical sensors offers significant sensitivity, the relatively large size of the additional polarizer and phase plate has severely restricted the application of such arrangements. An optical fiber sensor that utilizes SPR to detect a material in contact with the sensor and utilizes incident light having multiple wavelengths as the excitation energy is described by Jorgenson, et al. (U.S. Pat. No. 5,359,681). While being small and considerably less expensive than the non-waveguide optical sensor, it is at least an order of magnitude less sensitive. The reason for this drop in sensitivity is obviousxe2x80x94in a non-waveguide optical scheme with 5000 pixels, the SPR minimum is read by at least 2000 pixels. However, in the fiber optic scheme with a spectrometer as a readout instrument (wavelength resolution not better than 0.1 nanometer and SPR minimum spectral width around 60 nm) the SPR minimum will be characterized by 600 pixels at most, leading to less precise interpolations to locate the minimum, and hence changes in the wavelength position of the minimum. Conversely, the use of inexpensive tunable lasers with sub-nm wavelength resolution as light sources in fiber systems will eliminate an expensive spectrometer and yield precision at least comparable to that of the non-waveguide optical system.
The illustrative problem that arises in the broadband light source/spectrometer configuration, specifically a lack of optical intensity per measurement point, has, in the case of SPR sensors, the additional undesirable attribute of overheating of the sensor. If the total light intensity of the broadband light source is increased to compensate for the small intensity available to each pixel in the spectrometer charge coupled device (CCD) array, overheating of the SPR sensor will occur because approximately half the incident light is dissipated in heat in the metal layer whether it contributes to the usable signal or not. Heating is not only often harmful to the biological and/or chemical sample under test, but also can induce refractive index changes in the fiber and/or sample, causing much larger variations in the SPR wavevector than the perturbation to be detected. Very fast tuning lasers, having very narrow emission spectra, are ideal to address this problem, since the total intensity illuminating the sensor at any given time will be almost exactly equal to the intensity of the reflected light available to detect the change in stimulus.
An illustrative example of a characteristic absorber/reflector material is a semiconductor. A semiconductor-based optical sensor with optical-wavelength-dependent characteristics that may vary as a function of a physical parameter such as temperature is well known to those skilled in the art (see, for example patents, issued to Christenson (U.S. Pat. No. 4,136,566) or Quick et al. (U.S. Pat. No. 4,355,910)). The optical-wavelength-dependent characteristic (semiconductor absorption band edge) is usually monitored in one wavelength band, in which case measurements are intensity-dependent, or in two wavelength bands, after which a ratio of intensities is taken. In both cases, the sensitivity and accuracy of such sensor systems are low and more-or-less sensitive to optical noise (microbending, etc.). Scanning very rapidly through the whole semiconductor transmission intensity slope related to the forbidden band edge using a tunable VCSEL with high wavelength resolution will provide the opportunity for mathematical enhancement of the sensitivity of such sensors by at least an order of magnitude. Broadband light source/spectrometer configurations are not suitable for reasons similar to those described above. The high total illumination intensity may likely cause self-heating of the sensor, which is detrimental especially for temperature sensors. Reducing the illumination intensity, on the other hand, will cause uncertainty due to photodetector dark noise and other sources of optical noise.
Fiber etalon-based sensors are well known to those skilled in the art (see, for example, U.S. Pat. No. 5,646,401 issued to E. Udd). Etalons consist of two mirrored surfaces that may be internal or external to the optical fiber. The reflectivity of an etalon is defined by interference between light waves reflected from first and second mirrors. The advantages of etalon-based pressure, temperature and/or stain sensors include the low cost of etalons and very high sensitivity. However, with broadband light sources used for interrogation, measurements that are intensity based or count interference fringes are very susceptible to optical noise or other technical problems (e.g., losing count of the fringes)xe2x80x94often to the point of being impractical. The sole practical, self-calibrating system uses an optical cross-correlating interferometer as a detector, also an expensive technique (see, for example, U.S. Pat. Nos. 5,202,939 and 5,392,117 both issued to Belleville, et al.).
A new kind of laser, a vertical cavity surface emitting laser (VCSEL), has recently been invented. Generally, VCSELs are made completely with wafer-level processing and the chips emit from the direction of the broad surface of the wafer, rather than having to be cleaved out of the wafer in order to have an exposed p-n junction edge from which to emit, as in older art. This enables another benefit to be designed into the wafer structurexe2x80x94tunability. This is done with micromachining (MEMS) technology by placing a stack of optical layers, forming a mirror, in front of the emitting surface in such a way that the stack can be varied in its distance from the emitting surface by piezoelectric, magnetic, electrostatic or some other microactuating means.
The groups of C. J. Chang-Hasnain (U.S. Patent, [IEEE J. on Selected Topics in Quantum Electronics, V 6, N 6, November 2000, p. 978]), J. S. Harris Jr. (U.S. Pat. No. 5291502, [Appl. Phys. Lett. 68 (7), February 1996 p. 891]), and Vakhshoori [Electronics Letters, May 1999, V. 35, N.11 p. 900] have shown the potential for making tunable VCSELs with MEMS tuning mechanisms with wide tuning ranges and fast tuning speeds. Tunable VCSELs are relatively simple to manufacture, exhibit continuous mode-hop-free tunability over a wide spectrum, and potentially offer orders of magnitude lower cost as compared to prior art tunable lasers or optical spectrometers. Integrated, MEMS-tunable VCSELS make possible truly affordable and accurate optical sensor systems by combining low cost detectors and low cost excitation sources, one or the other of which is very expensive in the prior art systems with the accuracy and resolution to be viable commercially. In addition to the orders of magnitude lower cost of source/detector combinations, lower cost sensor will become available because of the orders of magnitude greater tuning speed.
Exemplary embodiments of the present invention provide a means of optical wavelength scanning Bragg grating, characteristic absorber/reflector, etalon and surface plasmon resonance sensors of all types with integrated, MEMS-tunable VCSELs in order to measure various physical parameters at several orders of magnitude lower cost than prior art, with the added benefits of enhanced accuracy, ruggedness and reliability.
In more detail, exemplary embodiments of the present invention provide, as an illustrative embodiment, diagnostic systems which interface with optical fibers or optical waveguides having Bragg grating or other types of sensors as described herein, embedded therein for the determination of static and dynamic values of various physical, chemical or biological parameters, and, further, to provide means of guaranteeing wavelength accuracy during the scanning cycle.
In accordance with one aspect of a preferred illustrative embodiment of the invention, an optical sensor diagnostic system includes an integrated MEMS-tunable VCSEL for providing a wavelength-tunable light in response to a voltage or other control signal, the tunable light being launched into an optical waveguide. At least one optical sensor, disposed in the path of the tunable light, provides a reflected light having an associated local amplitude minimum, maximum or slope. The said local amplitude maximum could contain one or more local amplitude minimums inside said local amplitude maximum, while said local amplitude minimum could contain one or more local amplitude maximums inside said local amplitude minimum. The wavelength at said minimum, maximum or slope of amplitude varies in response to an environmental stimulus imposed upon the corresponding sensor.
The tunable VCSEL individually illuminates each of the sensors throughout its associated wavelength band of an amplitude minimum, maximum or slope. An optical circulation device, disposed in the path of the tunable light between the tunable VCSEL and the sensors, isolates the tunable VCSEL from the reflected light and directs the reflected light from each of the sensors to the optical detector means, disposed for detecting the reflected light and for providing an electrical detection signal indicative of the power of the reflected light. A tuning controller provides a variable voltage or other signal to the tunable VCSEL indicative of the desired wavelength of the tunable light. A signal processor responsive to the electrical detection signal interprets a shift in the wavelength of the magnitude minimum, maximum or slope due to the environmental stimulus, and provides a signal indicative of said stimulus.
According to another aspect provided by an illustrative embodiment of the present invention, an optical sensor diagnostic system includes an integrated MEMS-tunable VCSEL for providing a wavelength-tunable light in response to a voltage or other control signal, the tunable light being launched into an optical waveguide. At least one optical sensor, disposed in the path of the tunable light, provides a transmitted light having an associated local amplitude minimum, maximum or slope. The said local amplitude maximum could contain one or more local amplitude minimums inside said local amplitude maximum, while said local amplitude minimum could contain one or more local amplitude maximums inside said local amplitude minimum. The wavelength at said minimum, maximum or slope of amplitude varies in response to an environmental stimulus imposed upon the corresponding sensor. The tunable VCSEL individually illuminates each of the sensors throughout its associated wavelength band of an amplitude minimum, maximum or slope. An optical isolation device, disposed in the path of the tunable light between the tunable VCSEL and the sensors, isolates the tunable VCSEL from the reflected light. The light transmitted through the said at least one optical sensor is directed by an out-going fiber to the optical detector means, disposed for detecting the transmitted light and for providing an electrical detection signal indicative of the power of the transmitted light. A tuning controller provides a variable voltage or other signal to the tunable VCSEL indicative of the desired wavelength of the tunable light. A signal processor responsive to the electrical detection signal interprets a shift in the wavelength of the magnitude minimum, maximum or slope due to the environmental stimulus, and provides a signal indicative of said stimulus.
In accordance with one aspect of a preferred illustrative embodiment of the invention, the said optical sensors are of reflective Bragg grating type. The sensors reflect light, having maxima or minima inside the maxima at a different reflection wavelength for each sensor, which vary their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, electrical current or magnetic field imposed thereon.
In accordance with another aspect of a preferred illustrative embodiment of the invention, the said optical sensors are of a transmittive Bragg grating type. The sensors transmit light, having minima or maxima inside the minima at a different transmission wavelength for each sensor, which vary their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, electrical current or magnetic field imposed thereon.
In accordance with a further aspect of a preferred illustrative embodiment of the invention, the said optical sensors are of a reflective etalon type. The sensors reflect light, having maxima, minima or maxima and minima at a different reflection wavelength for each sensor, which vary their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, electrical current or magnetic field imposed thereon.
In accordance with a further aspect of a preferred illustrative embodiment of the invention, the said optical sensors are of a transmittive etalon type. The sensors transmit light, having maxima, minima or maxima and minima at a different transmission wavelength for each sensor, which vary their spectral positions due to an environmental stimulus, such as strain, pressure, temperature, current or magnetic field imposed thereon.
In accordance with a further aspect of a preferred illustrative embodiment of the invention, the said optical sensors are of a reflective Surface Plasmon Resonance type. The sensors reflect light, having maxima or minima at a different reflection wavelength for each sensor, which vary in their spectral positions due to an environmental stimulus, such as temperature, biological or chemical stimuli imposed thereon.
In accordance with a further aspect of a preferred illustrative embodiment of the invention, the said optical sensors are of a transmittive Surface Plasmon Resonance type. The sensors transmit light, having maxima or minima at a different reflection wavelength for each sensor, which vary in their spectral positions due to an environmental stimulus, such as temperature, or biological and chemical stimuli imposed thereon.
In accordance with a further aspect of a preferred illustrative embodiment of the invention, the said optical sensors are of a characteristic absorber/reflector type. Said characteristic absorber/reflector sensors, disposed in the path of the tunable light, provide a transmitted light having an associated local amplitude slope or local amplitude minimum, the wavelengths of which vary their spectral position due to an environmental stimulus, such as temperature, imposed thereon. This embodiment could be also realized in reflection mode, with multiple sensors coupled off the main fiber, if the reflective means is disposed in the light path such that the light is double-passed through each of the sensors by means of a mirror and time domain multiplexing. In the case of an absorber/reflector exhibiting a minimum, wavelength division multiplexing can be utilized to the degree the width of the minimum allows as a fraction of the available tuning spectrum. The isolator in this realization of the present embodiment may be replaced by a circulator means.
In one example arrangement, the sensor produces a characteristic absorption feature in the form of a slope, wherein the said wavelength indicative of the characteristic absorption slope is determined by taking the first derivative of the light amplitude with respect to the wavelength, and by analytically extracting the wavelength position resulting from said first wavelength derivative extremum. In another example arrangement, the sensor produces a characteristic absorption feature in the form of a slope, wherein the said wavelength indicative of the characteristic absorption slope is determined by taking the second derivative of the light amplitude with respect to the wavelength, and by analytically extracting the wavelength positions of said second wavelength derivative zeros.
The illustrative embodiments of the invention provide low cost, workable, practical diagnostic systems which function in cooperation with remote optical fiber sensor systems to measure static and dynamic strain, pressure, temperature, electrical currents and magnetic fields as well as acoustic or vibratory perturbations of items or structures and chemical and biological parameters. The remote sensors may be disposed on structures made of metal, plastic, composite, or any other materials that expand, contract, or vibrate, or the sensors may be embedded within such structures or immersed in liquids or gasses.
The embodiments also provide a wavelength-tunable VCSEL, tunable smoothly and monotonically, and in particular, linearly or sinusoidally tunable with time. The embodiments further provide individual illumination of each sensor, thereby allowing all the tunable VCSEL power to be resident in a single narrow wavelength band at any instant in time. As a result, the reflected or transmitted light from each optical sensor has a high intensity, thereby providing a signal-to-noise ratio of such reflected or transmitted light that is much greater than systems that illuminate all sensors at the same time using a broadband source.
Ultra-fine tuning of tunable VCSELs to a few parts per million will allow another order of magnitude increase in precision due to higher resolution and improved computational methods and statistical processing. The very low mass of the MEMS tuning mechanisms allow very high tuning speeds with very low hysteresis, providing the ability to average out optical noise in the sensor systems with many data points and allowing very close spacing of data in wavelength.